Bump requests from 2.18.2 to 2.20.0 in /requirements

[dependabot]

Bumps requests from 2.18.2 to 2.20.0.

Changelog

Sourced from requests's changelog.

2.20.0 (2018-10-18)

Bugfixes

• Content-Type header parsing is now case-insensitive (e.g.
  charset=utf8 v Charset=utf8).
• Fixed exception leak where certain redirect urls would raise
  uncaught urllib3 exceptions.
• Requests removes Authorization header from requests redirected
  from https to http on the same hostname. (CVE-2018-18074)
• should_bypass_proxies now handles URIs without hostnames (e.g.
  files).

Dependencies

• Requests now supports urllib3 v1.24.

Deprecations

• Requests has officially stopped support for Python 2.6.

2.19.1 (2018-06-14)

Bugfixes

• Fixed issue where status_codes.py's init function failed trying
  to append to a __doc__ value of None.

2.19.0 (2018-06-12)

Improvements

• Warn user about possible slowdown when using cryptography version
  < 1.3.4
• Check for invalid host in proxy URL, before forwarding request to
  adapter.
• Fragments are now properly maintained across redirects. (RFC7231
  7.1.2)
• Removed use of cgi module to expedite library load time.
• Added support for SHA-256 and SHA-512 digest auth algorithms.
• Minor performance improvement to Request.content.
• Migrate to using collections.abc for 3.7 compatibility.

Bugfixes

• Parsing empty Link headers with parse_header_links() no longer
  return one bogus entry.

... (truncated)

Commits

• bd84045 v2.20.0
• 7fd9267 remove final remnants from 2.6
• 6ae8a21 Add myself to AUTHORS
• 89ab030 Use comprehensions whenever possible
• 2c6a842 Merge pull request #4827 from webmaven/patch-1
• 30be889 CVE URLs update: www sub-subdomain no longer valid
• a6cd380 Merge pull request #4765 from requests/encapsulate_urllib3_exc
• bbdbcc8 wrap url parsing exceptions from urllib3's PoolManager
• ff0c325 Merge pull request #4805 from jdufresne/https
• b0ad249 Prefer https:// for URLs throughout project
• Additional commits viewable in compare view

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